Re: [GIT PULL v2 1/5] processor.h: introduce cpu_relax_yield

Russell King - ARM Linux
2016-11-15T13:37:58Z

On Tue, Nov 15, 2016 at 02:19:53PM +0100, Christian Borntraeger wrote:
> On 11/15/2016 01:30 PM, Russell King - ARM Linux wrote:
> > On Tue, Oct 25, 2016 at 11:03:11AM +0200, Christian Borntraeger wrote:
> >> For spinning loops people do often use barrier() or cpu_relax().
> >> For most architectures cpu_relax and barrier are the same, but on
> >> some architectures cpu_relax can add some latency.
> >> For example on power,sparc64 and arc, cpu_relax can shift the CPU
> >> towards other hardware threads in an SMT environment.
> >> On s390 cpu_relax does even more, it uses an hypercall to the
> >> hypervisor to give up the timeslice.
> >> In contrast to the SMT yielding this can result in larger latencies.
> >> In some places this latency is unwanted, so another variant
> >> "cpu_relax_lowlatency" was introduced. Before this is used in more
> >> and more places, lets revert the logic and provide a cpu_relax_yield
> >> that can be called in places where yielding is more important than
> >> latency. By default this is the same as cpu_relax on all architectures.
> >
> > Rather than having to update all these architectures in this way, can't
> > we put in some linux/*.h header something like:
> >
> > #ifndef cpu_relax_yield
> > #define cpu_relax_yield() cpu_relax()
> > #endif
> >
> > so only those architectures that need to do something need to be
> > modified?
>
> These patches are part of linux-next since a month or so, changing that
> would invalidate all the next testing. If people want that, I can certainly
> do that, though.

It's three weeks since you posted them. For one of those weeks (the
week you posted them) I was away, and missed them while catching up.
Sorry, but it sometimes takes a while to spot things amongst the
backlog, and normally takes some subsequent activity on the thread to
bring it back into view.

--
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.